1. Field of the Invention
The present invention relates to a workpiece machining apparatus for machining workpieces by detecting the position thereof by reflection light from position-detecting through holes formed through the workpiece.
2. Description of Related Art
Hitherto, positioning of a printed circuit board, i.e., a workpiece to be machined by a workpiece machining apparatus, on a table has been carried out based on an alignment mark provided on a surface or in an inner layer of the printed circuit board. Then, there has been a method of irradiating light from the under (bottom) side of the printed circuit board to detect the alignment mark of the printed circuit board whose insulating layer is transparent or semitransparent in silhouette as disclosed in Japanese Patent No. 3023320.
Lately, it has become possible to machine a blind hole for connecting an upper layer with an inner layer by laser even in a printed circuit board whose surface layer (uppermost layer) is a copper layer. In this case, a surface treatment of forming a black CuO layer for example is often implemented on the surface of the uppermost copper layer to prevent the laser from being reflected by the copper layer. Then, a through hole is adopted as an alignment mark because the copper layer does not transmit light and it is hard to dispose an alignment mark on the surface by printing or the like.
FIG. 6 shows a prior art method for detecting the alignment mark by scanning an image thereof and FIG. 7 shows a printed circuit board P seen from a direction of an arrow K in FIG. 6.
An uppermost layer of the printed circuit board P is a copper layer and a black CuO layer whose thickness is about 2 μm is formed on the surface thereof. A thickness of the printed circuit board P is 1 mm to 3 mm. A plurality of through holes 18 (referred to as “alignment marks” hereinafter) is formed through the printed circuit board P. A diameter d1 of the alignment mark 18 is 1 mm to 3 mm. A ringed (annular) light 22 is disposed at an outer periphery of a camera 20. The camera 20 is connected with an image processing device not shown.
When the alignment mark 18 is illuminated aslant, a profile of the alignment mark 18 becomes ambiguous because a shadow S is made due to the illumination within the alignment mark 18. Because the image processing device calculates center coordinates of the alignment mark 18 by detecting the profile of the alignment mark 18, its precision for detecting the center coordinates drops when the profile detecting precision drops. Then, machining precision of the workpiece machining apparatus has dropped as a result. The profile detecting precision drops when a scratch is made around the alignment mark 18 in particular.
It may be possible to solve the abovementioned problem by adopting the technique of Japanese Patent No. 3023320. However, a degree of freedom of disposition of the alignment marks decreases when a number of illuminators is reduced. When the number of illuminators is increased to increase the degree of freedom of disposition of the alignment marks, there arise problems in that not only does the structure of the apparatus become expensive, but also a space for disposing vacuum chucking holes becomes narrow. It thus limits a number of holes for chucking the printed circuit board P and lowers a workpiece holding performance.
Accordingly, an object of the invention is to provide a workpiece machining apparatus whose workpiece machining precision is improved.